Color imaging of parietal and frontal somatosensory potential fields evoked by stimulation of median or posterior tibial nerve in man.

Somatosensory evoked potentials (SEPs) to median or fingers or posterior tibial nerve stimulation were recorded with earlobe reference in normal young adults. A system of 16 electrodes on the scalp served to create bit-mapped images of the potential fields at 1 msec intervals. The P14 (median SEP) or P30 (tibial SEP) far fields thought to reflect the afferent volley in the medial lemniscus produced widespread positivity over the scalp. Subsequent components had a characteristic focal distribution suggesting that they reflected one or more generators in cortical areas. For the median SEP, the parietal N20 and the prerolandic P22 showed differences in onset and offset times as well as distribution that precluded their being related to the same generator. While N20 was contralateral, P22 extended ipsilaterally. P22 may be generated in the motor area 4 and the supplementary motor area. P22 was also distinct from the P27 field restricted to the contralateral parietal region. The frontal N30 had a bilateral distribution and the P45 presented variable features. For the tibial SEP, no phase reversal was confirmed between the parietal P38 (midline-ipsilateral focus) and N33 (contralateral focus). N37 over the contralateral prerolandic region might reflect a generator in the motor region. P58 was more symmetrically distributed than P38, possibly because it reflected generators more posteriorly on the parietal convexity. N75 had a widespread field with focus on the ipsilateral side of midline.